1. Field of the Invention
The present invention relates to a lithographic printing plate precursor and a lithographic printing method using the same. Specifically, the invention relates to a lithographic printing plate precursor capable of direct plate-making by scanning with infrared laser beams on the basis of digital signals of, e.g., a computer, i.e., a so-called direct plate-making lithographic printing plate precursor, and also relates to a lithographic printing method of directly developing the lithographic printing plate precursor on a printing press and performing printing without going through development process.
2. Background Art
A lithographic printing plate generally comprises a lipophilic image area that receives ink and a hydrophilic non-image area that receives a fountain solution in printing. Lithographic printing is a printing method of making difference in ink-adhering property on the surface of a lithographic printing plate with the lipophilic image area of the lithographic printing plate as the ink-receptive area and the hydrophilic non-image area as the fountain solution-receptive area (ink-repellent area) by making use of the natures of water and oil ink of repelling to each other, adhering ink only on the image area, and transferring the ink to the material to be printed, e.g., paper.
For manufacturing this lithographic printing plate, a lithographic printing plate precursor (a PS plate) comprising a hydrophilic support having provided thereon a lipophilic photosensitive resin layer (an image-recording layer) has so far been widely used. The lithographic printing plate is generally obtained by a plate-making method of exposing a lithographic printing plate precursor through an original image of a lith film and the like, and then, for leaving the image-recording layer of the image area behind, dissolving and removing the image-recording layer of the non-image area with an alkali developing solution or an organic solvent, to thereby bare a hydrophilic support surface.
In a conventional plate-making process of a lithographic printing plate precursor, a process of dissolving and removing a non-image area with a developing solution and the like corresponding to the image-recording layer after exposure is necessary, but the exclusion or simplification of such an additional wet process is one of the objects in the industry. Since the discard of waste solutions discharged with wet processes is a particularly great interest in the industry at large in recent years from the consideration of the global environment, the solution of the above problem is increasingly desired.
Concerning this requirement, as a simple plate-making method, a method that is called on-press development is proposed, which is a method of using an image-recording layer capable of being removed a non-image area of a lithographic printing plate precursor in an ordinary printing process, and removing a non-image area after exposure on a printing press to obtain a lithographic printing plate.
As the specific examples of on-press development, e.g., a method of using a lithographic printing plate precursor having an image-recording layer soluble or dispersible with, e.g., a fountain solution, an ink solvent, or an emulsified product of a fountain solution and ink, a method of mechanically removing an image-recording layer by the contact with the rollers and the blanket of a press, and a method of mechanically removing an image-recording layer by the contact with the rollers and the blanket after weakening the cohesive strength of the image-recording layer or the adhesive strength of the image-recording layer and a support by the permeation of a fountain solution and an ink solvent are exemplified.
In the present invention, unless otherwise indicated, “development process” means a process of removing the area of an image-recording layer of a lithographic printing plate precursor not irradiated with an infrared laser by being brought into contact with a liquid (generally an alkali developing solution) to thereby bare the hydrophilic support surface with an apparatus other than a printing press (an automatic processor, in general), and “on-press development” means a method and a process of removing the area of an image-recording layer of a lithographic printing plate precursor not irradiated with an infrared laser by being brought into contact with a liquid (generally printing ink and/or a fountain solution) to thereby bare the hydrophilic support surface with a printing press.
However, when a conventional image-recording layer of an image-recording system utilizing ultraviolet rays and visible rays is used, it is necessary to take methods requiring much labor, such that the exposed lithographic printing plate precursor must be stored under a completely light-shielding condition or a constant temperature condition until it is mounted on a printing press, since the image-recording layer is not fixed after exposure.
On the other hand, in recent years, digitized techniques of electronically processing, accumulating and outputting image data using a computer have prevailed, and various image output systems corresponding to these digitized techniques have been put to practical use. Under such circumstances, a computer-to-plate technique of directly making a printing plate is attracting public attention, which comprises scanning exposing a lithographic printing plate precursor with high convergent radiant rays such as laser beams carrying digitized image data without using a lith film, With such a tendency, it is an important technical subject to obtain a lithographic printing plate precursor well adapted to this purpose.
Accordingly, in recent years, the simplification of plate-making operation, and the realization of dry system and non-processing system have been more and more strongly required from both aspects of the above-described global environmental protection and the adaptation for digitization.
Since high output lasers such as semiconductor lasers and YAG lasers radiating infrared rays of the wavelength of from 760 to 1,200 nm are inexpensively available nowadays, methods of using these high output lasers as the image recording means are now promising as the manufacturing method of a lithographic printing plate by scanning exposure that is easy to be integrated in digitized techniques.
In conventional plate-making methods, a photosensitive lithographic printing plate precursor is imagewise exposed by low to middle intensity of illumination, and image recording is performed by the imagewise changes of physical properties by photochemical reaction in the image-recording layer. While in the above method of using high output lasers, an exposure area is irradiated with a great quantity of light energy in an extremely short period of time to efficiently convert the light energy to heat energy, the heat energy is used to cause heat changes such as chemical changes, phase changes and morphological or structural changes in the image-recording layer, and these changes are utilized in image recording. Accordingly, image data are inputted by light energy, e.g., laser beams, but image recording is performed in the state including the reaction by heat energy in addition to light energy. A recording system making use of beat generation by such high power density exposure is generally called heat mode recording, and converting light energy to heat energy is called light/heat conversion.
Great advantages of the plate-mating method using beat mode recording are that image-recording layers are photo-insensitive to the lights of ordinary levels of illuminance such as room illumination, and that the fixation of images recorded by high illuminance exposure is not necessary. That is, lithographic printing plate precursors for use in heat mode recording are free of sensitization by room illumination before exposure and the fixation of images is not essential after exposure. Accordingly, a printing system that an image is not influenced even if exposed to room light after exposure becomes possible by using, e.g., an image-recording layer which is solubilized or insolubilized by exposure with high output laser beams and performing plate-making process by on-press development to make an exposed image-recording layer to an imagewise lithographic printing plate. Therefore, it is expected that a lithographic printing plate precursor preferably used for on-press development will be possible to be obtained if heat mode recording is used.
As one example concerning this mode, a lithographic printing plate precursor comprising a hydrophilic support having provided thereon an image-forming layer containing hydrophobic thermoplastic polymer particles dispersed in a hydrophilic binder is disclosed in patent literature 1 (Japanese Patent 2938397). The patent literature 1 discloses that it is possible to perform on-press development with a fountain solution and/or ink by subjecting the lithographic printing plate precursor to exposure with infrared laser beams to coalesce the hydrophobic thermoplastic polymer particles by beat to thereby form an image, and then mounting the lithographic printing plate precursor on the cylinder of a printing press.
However, although a method of forming an image by coalescence of fine particles by mere heat fusion as above certainly shows good on-press developing properties, there are problems that image strength (the adhesion of an image-forming layer and a support) is extremely weak and press life is insufficient.
Further, lithographic printing plate precursors having an image-recording layer (a heat-sensitive layer) containing microcapsules encapsulating a polymerizable compound on a hydrophilic support are disclosed in patent literature 2 (JP-A-2001-277740 (The term “JP-A” as used herein refers to an “unexamined published Japanese patent application”.)) and patent literature 3 (JP-A-2001-277742).
Further, patent literature 4 (JP-A-2002-287334) discloses a lithographic printing plate precursor comprising a support having provided thereon an image-recording layer (a heat-sensitive layer) containing an infrared absorber, a radical polymerization initiator and a polymerizable compound.
These methods of using a polymerization reaction are characterized in that relatively high image strength can be obtained, since chemical bonding density of an image area is high as compared with an image area formed by coalescence of polymer fine particles. However, from the practical point of view, any of on-press developing properties, fine line reproducibility and press life is insufficient and these systems have not been put to practical use yet.
Further, a lithographic printing plate precursor capable of on-press development comprising a support having thereon an image-recording layer containing a polymerizable compound, a graft polymer having polyethylene oxide chains on the side chain or a block polymer having a polyethylene oxide block is disclosed in patent literature 5 (U.S. patent application publication 2003/0,064,318).
However, according to this technique, good on-press developing properties can be obtained but fine line reproducibility and press life are still insufficient.